A108216 - OEIS

%I #9 Mar 20 2015 02:49:48

%S 3,3,4,5,2,4,3,2,4,4,1,4,4,2,5,3,3,2,3,1,6,6,2,2,2,3,3,2,2,4,5,2,5,3,

%T 2,2,3,2,3,5,2,4,2,1,3,4,2,3,4,2,3,5,3,4,3,4,3,2,4,2,0,2,4,3,1,1,3,3,

%U 4,5,3,3,2,3,3,3,3,3,3,4,3,4,0,3,4,1,4,3,2,4,2,3,4,3,2,4,2,3,3,3,3,2,1,1,5

%N Number of semiprimes between 10n and 10n + 9.

%C a(60) = a(82) = a(142) = 0. - _Jonathan Vos Post_, Jun 16 2005

%e a(0) = 3 because between 0 and 9 there are three semiprimes: 4, 6 and 9.

%e a(1) = 3 because between 10 and 19 there are three semiprimes: 10, 14 and 15.

%t f[n_] := Sum[ PrimePi[n/Prime[i]] - (i - 1), {i, PrimePi[ Sqrt[n]]}]; Table[f[10n + 9] - f[Max[10n - 1, 0]], {n, 0, 104}] (* _Robert G. Wilson v_, _Ray Chandler_ *)

%Y Cf. A038800 number of primes between 10n and 10n+9.

%Y Cf. A064911, A066265, A072000.

%K nonn

%O 0,1

%A _Giovanni Teofilatto_, Jun 16 2005

%E Edited and extended by _Ray Chandler_, Jul 07 2005